The use of mechanical insufflation and exsufflation (MI-E) with negative pressure is a well-known technique for helping patients with an ineffective cough to remove secretions from the respiratory tract. Patients who can benefit from the technique include: post-polio, muscular dystrophy, spinal muscular atrophy (SMA), post-cardiac surgery, amyotropic lateral sclerosis (ALS), mechanically ventilated, or anyone with insufficient muscle strength to generate the high expiratory flows necessary for moving secretions up the tracheobronchial tree. The technique involves the use of a blower and valve, which, via a facemask, mouthpiece or adapter for a tracheal tube, alternately applies positive pressure first to inflate the lungs, then shifts rapidly to negative pressure to create a high expiratory flow.
During automatic operation of the device, the internal valve executes a sequence of pressures applied to the patient's airway: first positive pressure to inflate the lungs, then a shift to negative pressure to create a high exhalation flow. This sequence is typically repeated a number of times (anywhere from 2 to 6) in succession for a treatment. The timing of each phase is adjustable by the user. Also, the initiation of the sequence is begun by the user actuating a switch. Alternatively, the user may also initiate each phase by actuating a switch while in a manual mode.
One shortcoming of current MI-E devices is that the beginning of an inhalation phase (positive pressure to first inflate the lungs) is triggered by either the patient or the patient's caregiver actuating a switch to begin the cycling. If the sequence is begun while the patient is exhaling, or the patient is not ready for an inhalation from the device, the patient may find the first inhalation uncomfortable, and may even unconsciously block the flow of air into the lungs. This can limit the effectiveness of the treatment, since a full deep inhalation breath is necessary to achieve adequate exhalation flow. Typically a caregiver must “coach” the patient during a treatment, explaining when to inhale, to avoid this problem. Alternatively, the caregiver watches the patient's respiration in order to switch on the cycling when the patient begins to inhale. Another shortcoming of the current device is that it is difficult to use on very young pediatric patients, and on unconscious or uncooperative patients, where it is difficult to explain to the patient when to begin an inhalation.
Assist modes have been used on positive pressure ventilators, which “breathe” a patient by applying a positive pressure to the airway, usually via an endotracheal or tracheostomy tube. Such assist modes also detect the patient's inspiratory effort in order to trigger the delivery of a breath by the ventilator. More recently, the advent of bi-level Continuous Positive Airway Pressure (CPAP) devices to support patients with respiratory insufficiency has also resulted in the use of some means to detect when the patient has begun inhalation, in order to determine when to increase the pressure applied to the airway.
While automatic assist modes work well with positive pressure ventilators they are not easily applicable to insufflation-exsufflation systems. In positive pressure ventilators the slight negative pressure or flow created by a patient inhalation is used to trigger a delivery of positive pressure, the breath or inhalation, for the patient. Then the positive pressure is stopped as the patient exhales to atmospheric or slight positive pressure. Following this the next negative pressure or flow sensed when the patient again inhales can be used to trigger delivery of the next positive pressure. In contrast in insufflation-exsufflation systems after the inhale/positive pressure comes the exhale negative pressure. After the time set for the negative pressure the system will return to the positive pressure but on the basis of the existing negative pressure which may not be the negative pressure created by a patient inhalation, thereby losing synchronism with the normal breathing of the patient.